This invention relates to a sorption pump, and in particular, to a sorption pump that is ideally suited for use in high vacuum gas analyzers.
A sorption or getter pump is described in U.S. Pat. No. 3,961,897 which issued to Giorgi et al. Getter pumps of this type are well known in the art and are used to establish and maintain a vacuum within a vessel. As noted by Giorgi et al. the sorptive material used in the pump may be selected from any number of non-evaporable getter materials that are suitable for pumping various gas molecules from the vessel in order to maintain the vessel at a desired vacuum pressure.
Giorgi et al., in a later U.S. Pat. No. 4,088,456, discloses a pumping system which, in its simplest form, is a cylindrical tube that is connected to a vacuum chamber. The internal surface of the tube is coated with a non-evaporable getter material characterized as having a sorptive capacity for certain gases at vapor pressures less than 10.sup.-5 torr. Suitable non-evaporable getter materials for this purpose are disclosed as Zr, Ti, Ta, Nb, V and mixtures thereof with other metals such as Al.
Sorption pumps containing getter materials absorb gas molecules either physically or through chemical reaction wherein the molecules are either captured on the material surface or dissolved into the material. In any event, the molecules are "pumped" from the vacuum chamber to maintain the chamber at a desired vacuum pressure. In the case of non-evaporating getters, the pump is able to form chemical compounds with most gases and thus can be used for a number of different applications.
Conventional sorption pumps, however, are not suitable for use in conjunction with gas analyzers and, in particular, in mass spectrometers. Common to all sorption pumps is the fact that the pumping speed, that is, the sorption speed of the getter material, is dependent on such variables as the getter material that is employed, the getter material history, the pump temperature, and the gas loading. The pumping speed can, and usually does, change dramatically with time and usage. In gas analysis equipment such as mass spectrometers, it is essential that one be able to accurately measure the partial pressure of a gas or gases involved. The partial pressure of a gas is expressed in terms of gas flow divided by pumping speed. Where the pumping speed is unknown or cannot be accurately determined, as in the case of a conventional getter pump, the gas flow also cannot be accurately evaluated quantitatively and the partial pressure cannot be determined.